This invention provides an improved method and apparatus for drying food and other commodities in a tumbling environment.
Rotary dryers are used today for drying of nuts and other commodities. In one commercial application, the nuts are introduced into a horizontal cylindrical drum, which is rotated about its horizontal axis to tumble the nuts. The drum is perforated, and hot gas from a conventional source, such as gas burners, is introduced from under the drum, flows through the perforations, and contacts the tumbling nuts for drying.
Inasmuch as drying depends on convection of hot gases past tumbling particles, the dryer suffers serious limitations. First, the rate of drying falls off after a portion of the moisture has been removed. The last few points of moisture removal take the longest and increase the cost of drying. If one attempts to increase the rate of moisture removal by increasing the temperature of the drying gas, the risk of overdrying or scorching the nuts becomes unacceptable.
The efficiency of drying is proportional to the temperature of the drying gas. However, product scorching or overdrying sets a practical upper limit for the gas temperature.
Most commercially dried food products have an empirically defined "safe" temperature, above which the risk of damaging the food particles with a conventional hot gas source becomes unacceptable. At or below this "safe" temperature, the product is protected from damage by a protective layer of moisture on its surface. The "safe" operating temperature is a wet bulb temperature and depends on this surface layer of moisture.
Heatless drying using ultrasonic energy has been used to dry slurries and other powdery materials. Sonic drying can be faster than drying performed by convection of hot gases and has the potential of increasing the capacity of drying systems. It is believed that the sonic energy removes the surface layer of moisture as soon as it can form on the particle. However, if sonic drying were to be combined with convection of hot gases, the particle would be exposed to the hot gas without the benefit of a protective surface film of moisture. Drying would occur at a dry bulb temperature. Such a proposed drying method would have to be carried out at a lower temperature than the empirically defined "safe" temperature in order to protect the product from damage. The efficiency of drying, which depends directly on the temperature of the hot gas, would suffer as a result.
One dryer using pulsating hot gas and sonic energy from a pulse jet engine is shown in U.S. Pat. No. 3,592,395, filed Sept. 16, 1968, to Lockwood et al. However, this dryer is a stirred fluid bed dryer which operates under different principles from rotary dryers and handles different products. The fluid bed dryer readily handles slurries or other fine powdery materials. However, the market pays a premium for recognizable pieces of food instead of powders. Any mechanically induced stirring will tend to break up the food pieces into smaller particles which are less valuable commercially. Moreover, mechanical stirring can damage delicate pieces.
The Lockwood fluid bed dryer has a set of rotatable stirring blades closely spaced above a horizontal floor. The blades rotate about an upright axis through the center of the floor. Hot pulsating gas and ground material in the form of a slurry enter the dryer at its center and flow under the blades toward an outer wall. Hot gas flows from under the blades to fluidize the bed. However, the gas is much hotter than the slurry and initially contacts a relatively small slurry volume. This tends to scorch or burn the product. Moreover, during start-up and shutdown, material sometimes tumbles down the centrally located hot gas inlet and causes fires. Furthermore, the fluidized bed tends to become nonuniform. The gas eventually geysers or erupts through some weak spot in the bed, and fluidization collapses.
High drying temperatures can promote product degradation by accelerating deleterious enzymatic and chemical processes. Efficiency must be sacrificed for product wholesomeness.
The unique structure of nuts presents an additional problem. The shell of the nut has a permeability different from the meat. An unsolved problem is to drive the moisture from both the meat and the shell economically and uniformly.
There is need for a rotary dryer which can remove moisture efficiently at low product drying temperatures, but at relatively higher gas dry-bulb temperatures.
There is also a need for a dryer which can safely combine hot gas convection and sonic energy at normal operating temperatures without damaging the product.